Tunable laser module for fibre optic communications
Espoo : VTT Technical Research Centre of Finland,
|Academic Dissertation to be presented with the assent of the Faculty of Technology, University of Oulu, for public discussion in Raahensali (Auditorium L10), Linnanmaa, on June 7th, 2004, at 12 noon.
This thesis deals with the design, realisation, and testing of the wavelength tunable 1540-nm laser modules that are intended for fibre optic communications. The short external cavity laser uses an electrically controlled silicon micromachined Fabry-Perot interferometer (FPI) device as the tuning element.
First, the most common techniques for the diode laser wavelength tuning and applications for these devices are reviewed. Then the ways of coupling laser power into the single-mode (SM) optical fibre are considered and the characteristics of the fibre attachment methods are studied.
The design, assembly and testing of three prototypes is introduced. Their discontinuous tuning spans range between 8 and 13 nm and their output power between 100 and 570 μW. The modules produce multimode tuning having an rms spectral width of about 1 nm. In addition, one prototype is capable of single-mode tuning with a side mode suppression ratio of 26 dB. The measured wavelength span is threefold compared to our earlier work near 980 nm and the widest reported single-mode tuning range obtained from a short external cavity Fabry-Perot diode laser without temperature tuning. This is also the first directly modulated external cavity laser that has a 3dB bandwidth of over 600 MHz.
The study confirms that it is possible to realise the laser-to-SM-fibre coupling using adhesive bonding. Further studies are needed in order to improve the speed and repeatibility of the assembly process together with the long-term reliability of the fibre attachment.
The prototype test results show that the FPI-tuned diode lasers have potential for applications where moderate performan ce suffices and low cost is mandatory. The hybrid arrangement uses standard laser components and therefore can provide a cost-effective and easily configurable solution for fibre optic communication and sensor applications. However, before the device characteristics can be optimised the theoretical understanding of the FPI-tuned laser operation must be enhanced. In addition, the packaging techniques of the modules need to be improved taking into account both the reliability requirements and the cost of production.